To fulfill the continuous growth of market demands, future wireless communications are required to have a very high data rate, a very large system capacity, a very low latency, etc. Among a number of technologies considered for these purposes, filter bank multicarrier (FBMC) transmission is a promising one, because this kind of scheme can lessen intersymbol interference (ISI) and intercarrier interference (ICI) without using guard intervals or cyclic prefixes and can achieve a higher spectral efficiency than orthogonal frequency division multiplexing (OFDM) transmission adopted by 4G wireless communication systems.
There are several FBMC approaches in the literature, including cosine modulated multitone, discrete wavelet multitone, filtered multitone, and OFDM using offset quadrature amplitude modulation (OFDM/OQAM), which is also referred to as FBMC/OQAM or staggered modulated multitone. Different from OFDM using quadrature amplitude modulation (OFDM/QAM) with each complex QAM data symbol modulated on a subcarrier during an OFDM frame duration, FBMC/OQAM transmits the real part and the imaginary part (including the imaginary “j” symbol) of each complex QAM data symbol on a subcarrier successively according to a staggering arrangement of timing offset that equals half an FBMC frame duration. Specifically, the real-part and imaginary-part data are placed on each subcarrier for FBMC/OQAM in a way that orthogonality among subcarriers and FBMC symbols holds in the real field, rather than in the complex field as with OFDM/QAM.
FBMC/OQAM adopts a time-frequency localized prototype (pulse shaping) filter with reduced sidelobes in both the time and frequency domains to lessen the ISI/ICI problems. Several types of prototype filters have been proposed for this purpose, including the root raised cosine function, half-cosine function, extended Gaussian function, isotropic orthogonal transform algorithm, and physical layer for dynamic access and cognitive radio (PHYDYAS) filter developed in an EU-funded research project.
Although an FBMC/OQAM system achieves excellent performance under single-input single-output (SISO) scenarios, conventional multiple-input multiple-out (MIMO) techniques such as Alamouti space-time block coding (STBC) and maximum likelihood detection cannot be applied directly, and some modifications along with complicated receivers are required. This is an undesired feature that limits applications of FBMC/OQAM.
For an FBMC/OQAM system with M subcarriers, a synthesis filter bank of a transmitter needs to perform two complex M-point inverse discrete Fourier transforms (IDFTs) for modulation and specific synthesis filtering operations in order to transmit a sequence of complex M-point QAM data symbols. On the other hand, an analysis filter bank of a receiver needs to perform two complex M-point discrete Fourier transforms (DFTs) for demodulation, specific analysis filtering operations, and appropriate data detection operations in order to receive a sequence of complex M-point QAM data symbols. Since this scheme uses the complex-valued IDFT and DFT, it is referred to as DFT-FBMC/OQAM.
To facilitate extension to MIMO scenarios, another kind of FBMC using IDFT/DFT and QAM, referred to as DFT-FBMC/QAM, was proposed recently, where a complete complex QAM data symbol is transmitted on a subcarrier during an FBMC frame duration. This kind of scheme adopts a set of two or more specific prototype filters, instead of a single prototype filter as used in DFT-FBMC/OQAM, to minimize self-interference. For example, with two orthogonal prototype filters, we can use one of them for even-numbered subcarriers and the other for odd-numbered subcarriers to effectively relieve the ISI/ICI problems. It was shown that DFT-FBMC/QAM using two prototype filters achieves slightly worse bit-error-rate (BER) performance than DFT-FBMC/OQAM, but is more easily combined with existing MIMO techniques.
For an FBMC/QAM system with M subcarriers and two prototype filters, a synthesis filter bank of a transmitter needs to perform two complex M/2-point IDFTs for modulation and specific synthesis filtering operations in order to transmit a sequence of complex M-point QAM data symbols. In contrast, an analysis filter bank of a receiver needs to perform two complex M/2-point DFTs for demodulation, specific analysis filtering operations, and appropriate data detection operations in order to receive a sequence of complex M-point QAM data symbols. Although this DFT-FBMC/QAM scheme involves complex-valued IDFT/DFT computations, its implementation complexity is lower than that of the DFT-FBMC/OQAM system.